Let’s make a list of iOS apps that satisfy either one of these criteria.

Open source.

Commercial app, but hardcore app for power users, not basic stuff.

The list should be biased towards gems, more obscure stuff that’s useful. Please don’t list apps that everyone knows about. I encourage you to post domain-specific apps, rather than general purpose apps.

WeatherPro, from the same company, the same data but in a different, more traditional format. Also for iPad.

Clear Outside, more tailored for outdoor stuff like hiking, nature photography, or astronomy. You get visibility, fog, wind, precipitation, and the types of clouds on the same, very easy to read screen.

I’ve always admired iOS for the strong commercial development scene that pays respect towards the platform and its users; imported from the Mac scene. Windows and Android are wastelands compared to the quality of small scale shops that put effort into it for iOS.

As for a recommendation, I haven’t had the oppurtunity to try the iOS version, but Coda is a great web development environment, if that’s how you roll.

Blink is a GPLv3-licensed SSH and Mosh client for iOS. It is pretty much what lets me travel light: With Blink I can travel without a laptop and use an iPad Pro with a smart keyboard to connect to and troubleshoot systems, or even do some limited development if it came to that. (The latter is more of a peace-of-mind thing, but it’s far from unpleasant.)

Warmly recommended, and easily worth the price if you don’t want to compile it yourself.

P.S: It’s almost embarrassing to say, but one of my favourite features is that it allows re-mapping caps lock to ctrl, which is usually not possible on iOS.

Blink also has more options for remapping certain keys on your keyboard, like “Caps as Esc”, “Shift as Esc”, etc. Although I have not purchased Prompt, since Mosh support for me is indispensable, these are other aspects people usually cite when comparing these two products.

I’ve never found a better app for another language period. Every serious Chinese learner will download this app after a year. No ads, extensive dictionary, writing input, long form text reader, powerful flash card engine, all for free. It’s so popular here in China among waiguoren that we even use Pleco as a verb.

German Grammar Spy spaced-repetition German article trainer. The words are sorted by frequency, so you learn the most common words first. I prefer this over normal vocabulary trainers because you can go through it very quickly.

Similar to previous app, Osmand, TIL it has iOS version too (was only for Android before)

Mapillary, allows to contribute to CC-BY-SA catalog of pseudo-street-view. Especially for those who miss Google’s Panoramio. Popular in Openstreetmap community because of free license of photos that allows to use them as reference data for editing map. Also they run interesting experiments with structure-from-motion, semantic segmentation and road sign detection on photos.

SongShift – playlist synchronisation between different music streaming services. Trained my Apple Music account in few weeks to.an acceptable level from Spotify input thanks to this.

I’m missing a good meditation app. I’ve tried Calm and Headspace, but they’re too featureful for me. I’d just want something that gives me a time and good selection of background noise/music for the duration of the session.